2 MECHANISMS UNDERLYING THE INDUCTION OF LONG-TERM POTENTIATION IN MOTOR CORTEX OF ADULT CAT IN-VITRO

Long-term potentiation of synaptic transmission (LTP), as documented b y the enhancement of evoked field potentials in layer III following st imulation of the underlying white matter, has been studied in slices o f motor cortex from adult cats. With a 1 M NaCl-filled recording elect rode, LTP was induced only in one out of eight slices. When the record ing electrode in addition contained 5 mM bicuculline metiodide, LTP wa s obtained with a much higher rate of success (15/19), suggesting that reduction of GABA, receptor-mediated inhibition facilitated the induc tion of LTP in the motor cortex. Bath application of DL-2-amino-5-phos phonovaleric acid (APV, 100 mu M) or Ni2+ (100 mu M) significantly red uced the success rate for LTP occurrence (6/16 and 5/16, respectively) ; but when LTP was induced, it did not show significant change in magn itude and time course. In slices perfused with APV (100 mu M) plus Ni2 + (100 mu M), LTP induction was completely blocked (0/12). These resul ts suggest that two different mechanisms may subserve LTP induction in the cat motor cortex: one is mediated by N-methyl-D-aspartate recepto rs and can be blocked by APV; the other may be mediated by low-thresho ld calcium channels and can be blocked by Ni2+.